Tectonic Evolution of the North Qaidam UHP Metamorphic Terrane, Western China

In this paper, we document the timing and structural development of the North Qaidam ultrahigh-pressure (UHP) terrane based on detailed field mapping and geochronology and thermochronology. The occurrence of UHP metamorphic rocks and their subsequent exhumation have been attributed to (1) continent-continent collision, (2) corner flow during continental or oceanic subduction, (3) diapiric flow of subducted continental materials, and (4) large-scale intracontinental thrusting. This particular case defies the conventional model of exhumation post-dating onset of continental collision, in that it appears to occur coeval with arc magmatism (~480-425 Ma) and lies within the arc. Our field mapping indicates that the Luliang Detachment fault is a regionally extensive feature placing lower amphibolite grade metamorphic rocks over the UHP rocks. This fault is currently folded due to an early phase of ductile deformation and more recent Cenozoic deformation. The cross-cutting relationships among the deformed and undeformed plutonic rocks and Ar/Ar thermochronology bracket the ductile deformation in the region between 480 and 430 Ma. The Luliang Shan detachment may be a Cordilleran style detachment or a shear zone accommodating the spreading of the UHP rocks once they emplaced to the lower crust. The two models imply very different rheological responses to UHP emplacement. The first requires the crust is to strong and coherent enough to allow discrete deformation, while the latter implies that the lower crust is weak. We prefer the latter model by noting that the UHP metamorphism occurred within an arc where presumably the regional crustal geotherm was high and the viscosity of the crust was low. This is consistent with the P-T-t paths constructed for the region that indicate that the temperature of North Qaidam UHP metamorphism increased with time.